In general, laser gyros or laser rate sensors operate under principle described by the Sagnac Effect by arranging for a laser to introduce two oppositely directed beams of light into a ring resonator. In a passive laser gyro, the light source is external to the ring resonator. In an active laser gyro, the light source is contained within the ring resonator. For reasons well described in the art, passive laser gyros are desirable over active laser gyros. Due to this fact, the operation of laser gyros will be described herein in terms of passive laser gyros.
The frequency of the laser is initially tuned so that an integral number of wavelengths fit within the length of the ring resonator. When the ring is at rest, such a frequency will cause the ring to be in resonance. When the ring goes through rotation about the axis perpendicular to the plane in which it lies, the apparent path length for the beam traveling around the ring in the direction of the rotation will become shorter than that of the beam traveling around the ring opposite direction of rotation. As the apparent path lengths of the rings being traveled by each of the two beams changes, the resonant frequency for each of the two beams will also change. By altering the frequency the beams, the rings being traveled by that beams can be brought back into resonance. The difference between the initial frequency required to establish resonance when the ring was at rest and the frequency required to re-establish resonance of the ring being traveled by one of the beams when the ring is being rotated, is directly related to the rate of rotation of the ring about the axis that is perpendicular to the plane in which it lies. Accordingly the rate of rotation of the ring can be derived from accurately measuring this difference in frequency.
The means currently described in the art for measuring the change in resonant frequency in such devices are known as closed loop laser gyros and open loop laser gyros. In the closed loop variety, the frequencies of both of the oppositely directed beams are independently controlled to maintain the rings being traveled by both of the beams in resonance concurrently. In the open loop variety, the frequency of the light source itself is controlled to maintain only the ring being traveled by one of the beams in resonance at any one time.
Closed loop laser gyros, and in particular U.S. Pat. No. 4,135,822 to Ezekiel and U.S. Pat. No. 4,326,803 to Lawrence, operate by concurrently altering the frequency of both of the beams being introduced into the ring resonator to maintain both of the counter-propagating beams in resonance. As such a frequency shifter is placed in the path of each of the beams to alter the frequency of each beam before it enters the ring. Each frequency shifter is responsive to the changes in resonant frequency produced by the changes in apparent path length of each of the beams. Each frequency shifter acts to bring the frequency of the beam passing through it into resonance for the apparent path length of the ring that it is traversing. The structure of the closed loop gyro necessary to maintain the frequency of each of the counter-propagating beams on resonance decreases the signal-to-noise ration, resulting in decreased signal quality. The necessary structure also adds to both the cost and complexity of the gyro. The increases in cost and complexity effectively foreclose the use of a closed loop laser gyro in many applications where such a device could otherwise be employed.
Open loop gyros, such as U.S. Pat. No. 4,674,881 to Lawrence, operate by stepping the frequency of the laser across the resonance frequency of the ring resonator in steps of a fixed time length. The differences in the intensity of light in one of the counter-propagating beams during the steps is detected. Electronics responsive to these differences are used to alter the level of the steps to drive the difference during the steps to zero. The intensity of light in the other counter propagating beam is then detected during the steps and the difference in intensity for the beam during the steps is determined. This difference is indicative of the rate of rotation of the ring. Open loop gyros, such as U.S. Pat. No. 4,661,964 to Haavisto, switch the frequency of the laser at a fixed rate of time between two values corresponding to the resonant frequencies for the apparent path lengths being traversed by each the two oppositely directed beams. The difference in these two values is then used to derive the rate of rotation of the ring. The structure necessary to accomplish the open loop system and more specifically the structure necessary to vary the frequency of the laser in fixed time steps between two corresponding frequencies results in an increase in cost and complexity that forecloses the use of existing open loop sensors in a number of applications where they would otherwise be suitable for use.
It is therefore an objective of the present invention to provide a simplified laser gyro without the need for the elements in the structure such as separate frequency shifting devices.
Yet another objective of the present invention is to provide a simplified laser gyro without the need for the elements in the structure that provide for biasing of the light source in fixed time intervals between two frequencies.
Yet another objective of the present invention is to provide a simplified laser gyro that is less complex to manufacture, and that may be produced at a reduced overall cost
Yet another objective of the present invention is to provide a simplified laser gyro that is less complex to manufacture, with a greater overall reliability
Yet another objective of the present invention is to provide a simplified laser gyro having less noise and therefore better signal quality.
Additional objectives, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations pointed out in the claims.
Still other objects and advantages of the invention will become readily apparent to those skilled in this art from the following detailed description of the preferred embodiment, wherein we have shown and described only the preferred embodiment of the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious aspects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.